Generally speaking, "vertical form, fill and seal" machines are well known and can be used, for example, to package milk in plastic pouches. More recently, such packaging has been used for other flowable materials, such as, mayonnaise, caramel, scrambled eggs, tomato ketchup, chocolate fudge, salad dressings, preserves and the like, particularly for the institutional user market, i.e., restaurants.
In operation, such "vertical form, fill and seal" machines will generally unroll a flat web of synthetic thermoplastic film and then form the film into a continuous tube by sealing the longitudinal edges with a lap seal or a fin seal. Generally, the tube is then moved downward to a station for filling. A sealing device below the filling station then creates an airtight heat seal across a transverse cross-section of the tube, using a pair of sealing jaws. The material (to be packaged) will generally enter the tube continuously, and therefore the film is generally sealed while material exists between the heat sealing surfaces in the tube.
After the sealing operation has been completed, the jaws are generally opened and the tube is then caused to move down a predetermined distance. Such downward movement may be influenced by the weight of the material in the tube, and/or by a drive mechanism in communication with the tube.
Once the tube moves down a predetermined distance, the heat sealing jaws collapse once again to create a second transverse seal. Almost simultaneously, the second traverse seal also severs the material-filled portion of the tube, thereby creating a sealed pouch of material. The second travsverse sealing operation also simultaneously creates the bottom seal for the next (to be formed) pouch. One such vertical form, fill and seal machine of the type described above is sold under the trade mark PREPAC.
Other conventional vertical form, fill and seal equipment cause the material (to be packaged) to enter the tube intermittently. In such cases, the material enters the tube only after the jaws have closed to form the first transverse seal. The jaws then open, and the tube is moved downward a predetermined distance. Then, before the second seal is made, the flow of material is stopped, so material will not locate between the heat sealing surfaces in the tube.
In other conventional machine designs, the sealing device does not sever the tube when making the second traverse heat seal, but rather, the tube is subsequently severed at a separate station.
With yet other machines, the heat sealing jaws move with the film as it moves down, and then releases the film at a predetermined distance. The jaws then move upward back to their original position to once again engage the film. With such machines, the jaws clamp, seal and sever the tube of film while moving in the downward direction. The jaws then open (and disengage from the film) and return to their original upward position. The downward movement of the closed jaws also serve to advance the tubular film downward.
The present invention relates to a heat sealing assembly for any of the above mentioned machines.
Conventional "impulse sealer" devices use short bursts of electrical current to create heat sealing temperatures during only a fraction of the cycle time between operations. The impulse sealer may be a round wire, e.g., a "piano" wire about 2.00 mm to 2.29 mm diameter, electrically insulated from a water-cooled supporting jaw. Alternatively, the impulse sealer can be rolled from wire stock into a flat ribbon having a longitudinal bead on the centre of one side (hereafter referred to as a "solid beaded element").
Impulse sealers having a round wire or solid beaded element are generally combined with conventional flat faced heat sealing jaws, and this design will generally be satisfactory for form and fill machines for packaging milk, water or other highly aqueous products. Other element shapes are generally more satisfactory on form, fill and seal machines when packaging thick flowable materials, such as, mayonnaise, chocolate fudge, scrambled egg mix, dressings, jams and the like. Examples of other conventional sealers are disclosed in U.S. Pat. No. 3,692,613, which issued to R. E. Pederson, U.S. Pat. No. 4,115,182, which issued to M. M. Wildmoser and U.S. Pat. No. 4,744,845, which issued to J. Posey.
Generally speaking, the heat sealing element must be electrically insulated from the metal jaw upon which it is mounted. Furthermore, the heat sealing element is also often thermally insulated from the jaw. Typically, this is accomplished by placing between the jaw and the heat sealing element, a woven glass cloth which is impregnated with polytetrafluoroethylene. The heat sealing element must be heated quickly when coming in contact with the film (to be sealed).
Generally, the jaw is wider than the flattened film, and the heat sealing element will extend beyond the edges of the jaws. The film will act as a heat sink at the "film contact" portion of the heat sealing element, and therefore, the heat sealing element tends to be hotter at the ends of the jaw, where the film does not come in contact with the heat sealing element.
As a result, the woven glass fibre cloth is continually subjected to high heat at the end portions of the heat seal element, where there is no film to drain the heat away. Thus the polytetrafluoroethylene often becomes heat degraded (i.e., burnt), and it is often necessary to replace the woven glass fibre cloth at frequent intervals. Previous attempts to alleviate this problem have not been entirely successful, and the present invention seeks to address this problem.